Twinning is increasing in the United States and elsewhere, and recent experience in the California Cerebral Palsy Project observed, and a subsequent study in Western Australia confirmed (both of these with an NEB scientist as participant) that cerebral palsy is more common in twins than in singletons and that cerebral palsy in twins now constitutes a larger proportion of all cerebral palsy than was true in the past. We undertook a study of twins in the NCPP because that dataset has certain advantages: good ascertainment of obstetric complications, recorded from the time of the first prenatal visit in twins and singletons, both those who turned out to have cerebral palsy or seizures and those who did not; information on zygosity on affected and unaffected twins; and information on several outcomes including in utero and neonatal death, and cerebral palsy, (CP) seizure disorders, and tested intelligence. In a total of 52,364 livebirths, there were 1079 twins. Although certain complications of pregnancy were more common in twins, they were not more common in twins with CP. Neonatal and febrile seizures occurred with similar frequency in twins as in singletons. Twins were much more likely to be low in birthweight, but twins <2500 g were not at higher risk for CP than singletons of that birthweight group, while in large twins the risk of CP exceeded that in large singletons. The risk of CP and of nonfebrile seizure disorders was similar in monozygous and dizygous pairs, implying that there must be pathogenetic mechanisms other than the disorders of placentation known to complicate monozygous twinning. Death of one twin was associated with higher rates of CP and of nonfebrile seizures in children without CP, but not with lower intelligence in those without CP or seizures. Low birthweight and in utero death of co-twin appear to be the dominant predictors of childhood neurologic disability in twins. A paper on this subject was published in this year.